Proposed Research

Groundwater discharge to coastal waters can have a significant
impact on local ecological structure because of its low salinity
and, often times, high nutrient content. Proper management of coastal
water resources requires that the quantity and quality of significant
inputs be adequately characterized. Groundwater input to coastal
waters, often termed submarine groundwater discharge (SGD), is particularly
difficult to characterize because it can vary significantly through
space and time. While hydrologic measurements can quantify fluid
flux at specific points, the spatial variability of flow can lead
to significant errors when making estimates using point data. Geochemical
tracers, on the other hand, are useful in quantifying total groundwater
flux to a surface water body, but are unable to identify the spatial
distribution of flow.

We propose to examine the utility of remote sensing technology,
specifically airborne thermal imaging, in identifying key groundwater
discharge locations to coastal surface water bodies. Hydrologic
measurements will be collected before and after the imaging, with
the sampling locations after the remote sensing work chosen based
on image results. We will also collect surface water and groundwater
samples for radium and dissolved nitrogen analyses. The radium samples
will be used to estimate groundwater discharge to the water bodies
and will be compared with the hydrologic estimates. Nitrogen samples
will be used, together with the flow estimates, to determine nitrogen
loading to the coastal waters. We propose to conduct the remote
sensing at three locations in southeastern New England: Quonochontaug
Pond in Rhode Island and Waquoit Bay and off the west coast of Truro
on Cape Cod. Previous work in these three locations indicates that
groundwater fluxes in these three areas vary by one to two orders
of magnitude.

Project Summary

In September, 2002, an aerial thermographic survey of Ninigret
Pond, (southern Rhode Island) and Waquoit Bay, (Cape Cod, Massachusetts)
was conducted. These images depict the surface temperature at the
time of the survey in (a) northwestern corner, (b)
Waquoit Bay; and (c) Ninigret Pond. Dark grey indicates cooler
temperatures and white represents warmer temperature. The dark grey
water adjacent to the coastline shows where cool groundwater is
discharging into the warmer surface water. In Waquoit Bay, groundwater
and surface water temperatures at the time of the survey were measured
in-situ as ~13° C and ~20-21° C, respectively.

Groundwater, flowing from the upland watershed to coastal embayment
can be a significant source of freshwater to coastal waters. This
groundwater carries retained chemicals and can be one source of
excess nutrient loadings in some ponds. Thus, groundwater may provide
a previously unanticipated load directly to the coast.

The shallow sediment (e.g., shallowest 10m) in Waquoit Bay consists
of medium to coarse sand and is generally regarded as having relatively
homogeneous properties. Based on this geology, we hypothesized that
groundwater discharge would be relatively uniform along the coast.
However, the Waquoit Bay image clearly shows that groundwater discharge
is not uniform. Instead, heterogeneities in the subsurface are likely
present and are responsible for the non-uniform discharge pattern.
Similarly, discharge to Ninigret Pond appears as point sources with
relatively low to no discharge occurring along most of the coast.
This non-uniform discharge will have a significant effect on estimates
of nutrient loadings to these ponds. In particular, the images show
that groundwater flux and nutrient loading estimates based on extrapolating
point measurements obtained without knowledge of the discharge pattern
may result in large errors. Finally, these images indicate that
additional work is needed to understand the effect of small-scale
geologic heterogeneity on groundwater discharge patterns.

This nighttime thermal infrared image
clearly identifies over 40 springs (bright white = warm groundwater)
that are upwelling in the water-body as well as several areas
of intense groundwater seepage along the shore. The data was
collected at night when the ground temperature was approximately
1.7°C and the groundwater was approximately 13°C. This
data was collected as part of a groundwater monitoring program
at an airforce base in the United States. This technique has
been proven to be very effective at surveying hundreds of square
miles to identify where groundwater is coming to the surface.

(a)

(b)

(c)

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